Piezoelectret microphones with high sensitivity

Improvements of the sensitivity of piezoelectret microphones based on charged cellular polymer films are reported. The improvements are achieved by (1) an increase of the piezoelectric d₃₃-coefficient of the cellular polypropylene films by pressure expansion and (2) stacking of the films. The pressure expansion results in an increase of the d₃₃-coefficient from about 150 pC/N to 600 pC/N in the audio frequency range. Microphones consisting of a single film of such material have sensitivities of about 2 mV/Pa at 1 kHz, independent of size, while for a microphone with five stacked films a sensitivity of 10.5 mV/Pa has been measured. The equivalent noise level is about 37 dB(A) for the single-film transducer and 26 dB(A) for the stacked version. Distortion is less than 1 percent at 164 dB SPL. Advantages of these new piezoelectric transducers include their simple design, consisting essentially only of the cellular film or films and electrical shielding. Other advantages are low cost, small weight, as well as a large range of shapes and sizes possible with such microphones.